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Reference surface shape influential in detecting early keratoconus
Sensitivity achieved using this parameter alone, however, is not sufficient and should be combined with other methods for detection.
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The best-fit toric and aspheric method of measuring corneal elevation shows promise for discerning subclinical keratoconus, according to a study.
“The modification of the reference surface shape, when analyzing the elevation maps, from the classical spherical shape to a toric and aspherical shape improved the sensitivity of the subclinical keratoconus detection,” lead study author David Smadja, MD, told Ocular Surgery News.
The study, published in the Journal of Refractive Surgery, assessed the influence of reference surface shape on corneal height measurements, comparing the best-fit toric and aspheric method of measurement with the more commonly used best-fit sphere method.
“By matching closer to the natural corneal shape, the best-fit toric and aspheric method would neutralize a part of the influence of the toricity and asphericity on the elevation map calculation and, therefore, would help to bring out the first signs of asymmetry in elevation,” Smadja said. “This feature becomes particularly relevant when tracking subtle abnormalities in elevation maps for detecting subclinical keratoconus.”
Patients and protocols
The prospective study included 391 eyes categorized into three groups: 167 eyes with keratoconus, 177 normal eyes and 47 topographically normal eyes of patients whose contralateral eye showed clinical signs of keratoconus.
All eyes were evaluated using the dual Scheimpflug analyzer (Galilei Analyzer, Ziemer Ophthalmic Systems). Maximum elevation values were collected and recorded in the anterior and posterior elevation maps within the central 5-mm diameter.
The investigators compared the discriminating ability of corneal elevation measurements obtained by the best-fit toric and aspheric method and the best-fit sphere method with that of receiver operator curves (ROC).
The posterior elevation calculated with the best-fit toric and aspheric method was significantly more accurate in predicting forme fruste keratoconus than that with the best-fit sphere method, with an area under the ROC of 0.88 compared to 0.59; however, it has also shown more accuracy than anterior elevation calculated with the best-fit toric and aspheric method, which achieved a ROC of 0.8, according to the study.
“The posterior surface elevation analyzed with the best-fit toric and aspheric method was shown to be significantly better than the anterior elevation for identifying the mildest forms of the disease, thus supporting the leading current hypothesis that keratoconic disease may be first detectable at the posterior surface,” Smadja said.
Sensitivity was also improved by using the best-fit toric and aspheric method; however, other relevant parameters should be used in conjunction with this fitting method for identifying subclinical keratoconus due to the method’s 18% false-negative rate for detecting forme fruste keratoconus, according to the investigators.
Alternative detection methods
In a study, slated to be published in the American Journal of Ophthalmology, Smadja and colleagues explore alternative methods for detecting subclinical keratoconus, including the use of artificial intelligence.
“We developed an automated detection program for identifying subclinical keratoconus using an artificial intelligence system,” he said. “This system, a decision tree classifier, was able to automatically select the most discriminate parameters and combination of parameters among 55 variables measured with the Galilei analyzer system, including elevation, curvature, aberrometric and pachymetric data for discriminating between normal corneas, subclinical keratoconus and keratoconus.”
Among the 55 parameters incorporated in the analysis for distinguishing between normal eyes and those with subclinical keratoconus, posterior asphericity asymmetry index and corneal volume were selected by the algorithm as the two most significant, Smadja said.
The asphericity asymmetry index is determined as the absolute value of the difference between the maximum negative elevation value and maximum positive elevation value within the 6-mm diameter data zone, according to a poster presented at the 2010 American Society of Cataract and Refractive Surgery meeting.
“Since this work, we always look at the asphericity asymmetry index of the posterior surface and the corneal volume before considering a cornea suitable or not for refractive surgery,” Smadja said. – by Christi Fox
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David R. Hardten, MD
This interesting study evaluates our ability to distinguish abnormal corneal curvature based on the shape of the reference shape. The normal cornea is a toric ellipsoid, and as such, in most normal corneas, there is very little variation from the best fit toric ellipsoid shape if the cornea is normal. This study compares the ability to distinguish abnormal eyes when a best fit sphere (the traditional comparison reference) with a best fit toric ellipsoid asphere. In my practice, I have been using this shape for comparison, and have also found it easier to distinguish normal from abnormal when using this shape, which is what the authors found. There are many increasingly useful methods being described to help us determine normal from abnormal corneas.
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Marianne O. Price, PhD
This study provides a valuable refinement of the methods used to differentiate between normal and subclinical keratoconus corneas when screening patients for laser refractive surgery. Posterior corneal elevation assessed by tomography is generally more sensitive and specific than anterior surface elevation assessed by topography, because corneal epithelial thickness variation can mask subtle protrusion of the anterior stromal surface. Typically, posterior elevation has been determined by comparing surface measurements with a best-fit sphere. However, this study shows that the sensitivity and specificity can be substantially improved by comparing the posterior surface with a best-fit toric ellipsoid shape that takes individual variation in cylinder and asphericity into account. With this refinement, the false negative rate dropped to 18% in eyes with subclinical keratoconus, so this technique is advised in combination with other assessment methods to identify those who might be at risk of developing ectasia.Click Here to Manage Email Alerts